MnS, MnS+V(C, N) complex precipitates in micro-alloyed ultra-fine grained steels were precisely analyzed to investigate the grain refining mechanism. The experimental results shows that MnS, MnS+V(C, N) precipitates provide nucleation center for Intra-granular ferrite (IGF), so that refined grain remarkably. Moreover, substructures such as grain boundary, sub-boundary, distortion band, dislocation and dislocation cell in austenite increased as the deformation energy led by heavy deformation at low temperature (deformation temperature≤800 ℃, deformation quantity≥50%). As a result, V(C, N) nanophase precipitated at these substructures, which pinned and stabilized substructures. The substructures rotated and transformed into ultra-fine ferrite. 20 nm-50 nm were the best grain size range of V(C, N) as it provided nucleating center for intragranular ferrite. The grain size of V(C, N) were less than 30 nm in the microalloyed steels that with volume ratio of ultra-fine ferrite more than 80% and grain size less than 4 μm.
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